The renin-angiotensin system has wide-ranging actions on numerous tissues in the body affecting blood pressure (pressor activity) and cardiovascular and electrolyte homeostasis. It is currently believed that angiotensins AII and AIII are derived via enzymatic cleavage in the cascade depicted in FIG. 1, steps 1, 2, and 3 (1). (Numbering herein of the amino acid residues in AI, AII, AIII, and AIV is according to that appearing in FIG. 1.) The resin-angiotensin cascade is thought to begin with the action of renin or angiotensinogen to release angiotensin I (AI), a biologically inactive decapeptide. Angiotensin II (AII), the bioactive octapeptide, is thought to be formed by the action of angiotensin converting enzyme (ACE) on circulating AI (2). Des-AspAII (Angiotensin III; AIII) is derived from AII, and certain reports have suggested possible activities for AIII in the adrenal gland (3) and brain (4). It has been reported that AII and AIII are inactivated by enzymatic degradation through a series of smaller inactive fragments (5). Fragments smaller than AIII have been though, for the most part, to be biologically inactive and of little physiological significance (6). This assumption has been based on the lack of pressor and certain endocrine activities (i.e., aldosterone release) of small angiotensin fragments (7) and the finding that N-terminal deleted fragments, i.e., smaller than AIII, reportedly exhibit low binding affinity for angiotensin AI or AII receptors (known as AT1 and AT2, respectively) as determined in radiolabeled ligand studies (8).
Certain studies have used AII.sub.(3-8) as one of several controls in structure-activity studies of AT1 and AT2 receptors (9,10). An AII receptor having components with molecular weights of 60-64 kDa and 112-115 kDa has reportedly been cloned from adrenal cortical cells as well as rat smooth muscle (11).
In general, AII.sub.(3-8) has been found to be much less active than AII or AIII with regard to typical angiotensin-dependent pressor activity or stimulating water intake (9,10,12). However, certain reports have suggested that AII.sub.(3-8), while having little pressor activity or ability to stimulate aldosterone release, may under certain circumstances inhibit renin release from kidney (12,13). Haberl et al. (14) reported a possible effect of AII.sub.(3-8) on endothelium-dependent dilation in rabbit brain. Braszko et al. (15,16) reported possible effects of AII.sub.(3-8) or AII.sub.(3-7) on motor activity, memory, and learning when administered intracerebroventricularly (icv) into rat brain and suggested that these effects should be considered "unspecific," i.e., not mediated by receptors (Braszko et al. (17), p. 195).
The angiotensin field has often been fraught with complexity and conflicting information, particularly with regard to the levels of different AII and AIII peptides required to elicit certain cellular responses, the concentrations predicted from receptor binding studies to be biologically active, and the levels of angiotensin peptides that may be measured in biological fluids. It has been reported that AII and AIII are removed from, or destroyed in, circulation by enzymatic hydrolysis. Biological half-lives of the different metabolic fragments are reportedly quite short. Semple and co-workers (18) reportedly detected AIII, AII.sub.(3-8), and AII.sub.(4-8) in arterial and venous blood in man with half-lives for AII, AIII, AII.sub.(3-8), and AII.sub.(4-8) of 4.4, 2.0, 1.9, and 2.4 minutes, respectively. Blumberg et al. (19) reported that during transit through the kidney 72-76% of AI and AII and 89% of AIII was metabolized.
Confusion has existed in the art as to how metabolic products of AII and AIII can exhibit certain biological activities (e.g., inhibition of renin release and enhancement of cognitive function), while failing to bind to AI or AII receptors. Fragments of AII smaller than AIII, e.g., AII.sub.(3-8) and other smaller fragments, have not been reported to have specific saturable binding sites in tissues, and receptors for these fragments have not been identified previously. The present invention provides partial explanation for certain previous confusing and contradictory findings, and provides novel AIV receptors (AT4), AIV ligands, peptides, analogs, agonists and antagonists that bind specifically to the AT4 receptor and not to AI (AT1) or AII (AT2) receptors. The AIV peptides and the AT4 receptor are labile and subject to proteolytic degradation. In other aspects, the invention provides a specific angioteninase enzyme that converts AII or AIII peptides to AIV peptides in a novel pathway.